I know what your problem is. Been a while since I dicked around with a stock carb, took me little bit to remember this.
Take the carb off the bike. Remove the float bowl, float, and accel pump rod. Remove the top "cap" and slide. All you are left with is the carb body, which is still holding the main and pilot jets, as well as the idle mixture screw. Now, this is where it get a little tricky. There is a Y shaped plastic insert in the carb. It goes in from the top, and is held in by 2 tamperproof torx screws (they have a little tit in the middle, so a standard torx driver won't work). This insert is what acts as a guide for the slide, and it is what the main jet screws into. Now, remove the two tamperproof torx screws from the top (where the slide goes in). Next, hold the carb with both hands, place the maim jet on a solid, flat, surface. Hold it damn steady, and press down as hard as you can until the plastic Y insert pops out. UNDER NO CIRCUMSTANCES USE PLIERS AND ATTEMPT TO PULL IT OUT FROM THE TOP!!!! It WILL break!!! It may take some doing, but, it will come out.
Once it is out, look at the bottom of the Y (where the main jet screws in), there will be two O-rings (or two grooves if the O-rings are not on there). Those O-rings go bad and allow excess fuel to get past the main jet, thus giving the symptoms you are having. Replace them and you will be good to go.
Apply a little penetration oil, or Vaseline to the them to ease installing the Y back into the carb body. Also, those O-rings are not your standard O-rings. They have square edges, so you may have to go to your hardware store and match them up. Lastly, be sure to blow the carb out with air REALLY well to make sure there are no o-ring particles stuck in there.
